Micromachining with short-pulsed, solid-state UV laser

ABSTRACT

In some embodiments, laser output including at least one laser pulse having a wavelength shorter than 400 microns and having a pulsewidth shorter than 1,000 picoseconds permits the number of pulses used to clean a bottom surface of a via or the surface of a solder pad to increase process throughput. An oscillator module in cooperation with an amplification module may be used to generate the laser output.

TECHNICAL FIELD

The invention relates to laser micromachining and, in particular, tolaser micromachining applications with a short-pulsed laser.

BACKGROUND OF THE INVENTION

Q-switched solid-state lasers are well known and have been demonstratedsuccessfully for many laser micromachining applications. However,micromachining parameters for Q-switched lasers, including theirwavelengths (ranging from near infrared to deep ultraviolet),pulsewidths, pulse energies, and pulse repetition rates, have still notbeen perfected for certain classes of layered organic, inorganic, andmetallic microelectronic material constructions with respect tothroughput and machining quality, such as cleanness, sidewall taper,roundness, and repeatability.

One such class of materials, commonly used in the printed wiring board(PWB) industry, includes glass cloth impregnated with one or moreorganic polymer resins that is sandwiched between conductive metallayers, typically copper. This material configuration is known as “FR4”or “BT.”

Another class, commonly used as packaging materials for high-performanceintegrated circuits, includes unfired, “green” ceramic materials. Theseceramic substrates are formed by high-pressure pressing of powders ofcommon ceramics such as aluminum oxide (Al₂O₃) or aluminum nitride(AlN). The micron- or submicron-scale particles are held together withorganic “binders” that provide sufficient mechanical integrity formachining operations such as via drilling. Afterward, the green materialis fired at high temperature, driving off the binders and fusing orsintering the microparticles together into an extremely strong, durable,high-temperature substrate.

U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describetechniques and advantages for employing Q-switched UV laser systems togenerate laser output pulses within advantageous parameters to formthrough-hole or blind vias through at least two types of layers inmultilayer devices, including FR4. These patents discuss these devicesand the lasers and parameters for machining them. These parametersgenerally include nonexcimer output pulses having temporal pulsewidthsof shorter than 100 nanoseconds (ns), spot areas with spot diameters ofless than 100 microns (μm), and average intensities or irradiances ofgreater than 100 milliwatts (mW) over the spot areas at repetition ratesof greater than 200 hertz (Hz).

U.S. Pat. No. 6,784,399 of Dunsky et al. discloses the use of aQ-switched carbon dioxide laser to produce bursts of laser pulses whosespikes and tails can be controlled to address disparate vaporizationtemperatures or melting points of the bulk via material.

U.S. Pat. No. 5,656,186 of Mourou et al. discloses a general method oflaser-induced breakdown and ablation at several wavelengths byhigh-repetition-rate ultrafast laser pulses, typically shorter than 10picoseconds (ps), and demonstrates creation of machined feature sizesthat are smaller than the diffraction limited spot size.

U.S. Pat. No. 5,742,634 of Rieger et al. discloses a simultaneouslyQ-switched and mode-locked neodymium laser device with diode pumping.The laser emits a series of pulses, each having a duration time of 60 to300 ps, under a time duration of 100 ns.

U.S. Pat. No. 6,574,250 of Sun et al. is the first to disclose a methodfor processing links on-the-fly with at least two laser pulses. Oneembodiment employs pulses having pulsewidths shorter than 25 picoseconds(ps).

U.S. Pat. No. 6,734,387 of Kafka et al. discloses the use of UVpicosecond laser output from a mode-locked, quasi-continuous wave (cw)laser system to cut or scribe lines in polymeric films.

SUMMARY OF THE INVENTION

An object of the present invention is, therefore, to provide a laserand/or method to increase throughput for laser micromachining ofmicroelectronic manufacturing materials.

Some preferred embodiments concern via drilling and/or ablation ofelectronic materials such as homogeneous films, particulate-filledresins, polyimides, and fiber-reinforced polymers, with or without metalcladding, using a picosecond pulsewidth solid-state UV laser.

Some preferred embodiments concern the machining of green ceramics,solder pad cleaning, or removal of photoresist material.

In some exemplary embodiments, the number of pulses employed to clean anunderlying pad is significantly reduced, and in most preferred casesonly one pulse is employed for cleaning.

In some embodiments, the laser output is generated by an oscillatormodule in cooperation with an amplification module. In some preferredembodiments, the oscillator module comprises a diode-pumped, solid-state(DPSS) master oscillator. In some preferred embodiments, the oscillatormodule comprises a pulsed semiconductor laser emitting picosecondpulses. In some embodiments, the oscillator module comprises a pulsedfiber master oscillator. In some preferred embodiments, the pulsed fibermaster oscillator comprises a diode-pumped, rare-earth-doped glass fibermaster oscillator employing a semiconductor saturable-absorbing mirror(SESAM). In some embodiments, the glass fiber master oscillatorcomprises a rare-earth-doped, fused silica fiber. The rare-earth dopantspreferably comprise Nd, Yb, Ho, Er, Dy, Pr, Tm, or Cr.

In some preferred embodiments, the amplification module comprises asingle-pass, multipass, or regenerative DPSS amplifier. In someembodiments, the amplification module comprises an Nd:GdVO₄, Nd:YVO₄,Nd:YLF, Nd:glass, or Nd:YAG gain medium. In some embodiments, theamplification module comprises a rare-earth-doped glass fiber poweramplifier. In some embodiments, the rare-earth-doped glass fiber poweramplifier comprises a rare-earth-doped fused silica fiber poweramplifier. The rare-earth dopants are preferably selected from Nd, Yb,Ho, Er, Dy, Pr, Tm, and Cr.

In some exemplary embodiments, imaged UV laser output having one or morepulses shorter than 1,000 ps is employed to perform the pad-cleaningprocess.

In some embodiments, the laser output comprises multiple independentlytriggered pulses or bursts of pulses selected from a pulse train emittedfrom the amplification module.

Additional objects and advantages of the present invention will beapparent from the following detailed description of preferredembodiments thereof, which proceeds with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of ablation depth versus number ofpulses for an exemplary via drilling.

FIG. 2 is a graphical representation of δN versus F/√{square root over(τ)} for different values of the parameter L.

FIGS. 3A and 3B are optical micrographs that show cross sections of viasdrilled in woven reinforced resin that exhibit respective small andlarge heat-affected zones.

FIGS. 4A and 4B are optical micrographs showing the results of processesto remove solder mask from pads that are approximately the same size asthe laser beam being used to remove the solder mask.

FIG. 5 is a simplified schematic diagram of an exemplary laser systemfor processing low-k dielectric material supported on a substrate.

FIG. 6 is a simplified partly pictorial and partly schematic diagram ofthe laser system of FIG. 5, showing some components of an exemplarybeam-positioning system.

FIG. 7 is a simplified pictorial diagram of an optional imaged opticsmodule that may be used in an exemplary laser system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments entail the use of a solid-state UV laser toperform via drilling and ablation of electronic circuit materials suchas homogenous films, particulate-filled resins, polyimides, andfiber-reinforced polymers, with or without metal cladding. An Ajinomotobuild-up film (ABF®) circuit-board dielectric material manufactured byAjinomoto Fine-Techno Co., Inc., Kawasaki, Japan, is typical of a targetmaterial upon which via drilling operations may be performed. Someexemplary workpieces include ABF® SH-9K, ABF® GX-3, ABF® GX-13, orsimilar products manufactured by other companies, but other via drillingtarget materials (including, but not limited to, multilayered, laminatedsubstrates such as those used in high-density printed wiring boards andintegrated circuit chip packages are also suitable for processing inaccordance with the exemplary embodiments disclosed herein.

Workpieces intended for via drilling typically contain conductivecladding layers that may be positioned on the top or bottom surfaces ofthe workpiece. These layers may contain, for example, standard metalssuch as aluminium, copper, gold, molybdenum, nickel, palladium,platinum, silver, titanium, tungsten, metal nitrides, or combinationsthereof. Conventional metal layers vary in thickness, typically between5 and 36 μm (where 7.8×10⁻³ kg of metal equals a thickness of about 9μm), but may be thinner, or as thick as 72 μm. The top and bottomconductive layers are typically made of the same material.

A dielectric matrix or layer positioned between the conductive layersmay contain a standard organic dielectric material such asbenzocyclobutane (BCB), bismaleimide triazine (BT), cardboard, cyanateesters, epoxies, phenolics, polyimides, polytetrafluorethylene (PTFE),various polymer alloys, or combinations thereof. Conventional organicdielectric layers vary considerably in thickness, but are typically muchthicker than the conductive layers. An exemplary thickness range for theorganic dielectric layers is about 30 to 400 μm.

The dielectric layer may also contain a standard reinforcement componentthat may be fiber matte or dispersed particles of, for example, aramidfibers, ceramics, or glass woven or dispersed throughout the organicdielectric layer and that may comprise much of its thickness.Conventional reinforcement components are typically individual filamentsor particles of about 1 to 10 μm and/or woven bundles of 10 μm toseveral hundreds of microns. Skilled persons will appreciate that thereinforcement components may be introduced as powders into the organicdielectrics and can be noncontiguous and nonuniform. Such composite orreinforced dielectric layers typically require laser processing at ahigher fluence than is needed to ablate unreinforced layers, but someparticle-filled resins can be processed at a fluence similar tounreinforced layers.

Some exemplary embodiments pertain to drilling blind vias, andparticularly drilling blind vias in homogeneous or filled resins. Suchblind via drilling is commonly done with a punching process, whereinsequential laser pulses are directed at a single target position on aworkpiece until a desired depth is achieved such that the bottom copperlayer is exposed.

In blind via drilling and analogous laser machining processes, a totalnumber of pulses N employed to form a qualified via includes a bulknumber N₀ of bulk removal pulses for bulk material removal and a bottomsurface cleaning number δN of cleaning pulses employed for cleaning offthe bottom (metal) surface or pad of the via. The number of pulses toclean a bottom metal pad can take the significant portion of the totalnumber of pulses required to drill the blind via if the laser pulsewidthis as long as several tens of nanoseconds. The bulk material removal andthe bottom cleaning involve different laser/material interactionmechanisms. So one efficient way to reduce the via drilling time wouldbe to reduce the bottom surface cleaning number δN of pulses used forbottom metal pad cleaning by adjusting laser parameters while somehownot adversely affecting the bulk material removal process.

FIG. 1 is a graphical representation of ablation depth versus number ofpulses for an exemplary via drilling punching process using an imaged UVbeam such that the total number of pulses N is partitioned into the bulknumber N₀ of bulk removal pulses and the bottom surface cleaning numberδN of cleaning pulses. FIG. 1 shows that depending on the applicationsand the solid-state UV laser sources being used, the cleaning number δNof pulses for bottom pad cleaning can be significantly different as δN₁,δN₂, δN₃, etc. For some applications, the ratio of δN to N₀ can be morethan 1, meaning that more time is spent cleaning the bottom pad than forbulk material removal. So it is desirable to reduce δN in order toreduce the total drilling time per via. It is also desirable to reduceδN in order to reduce the amount of energy dumped into a pad to avoidunnecessary thermal damage.

Conventional methods of controlling a via drilling process entailcontrolling the pulse energy for the given process. The pulse energy,E_(P), for use in performing a given process is determined by thefluence, F, desired for the process. The fluence, in J/cm², iscalculated as

$\begin{matrix}{{F = \frac{E_{P}}{\frac{\pi}{4}D^{2}}},} & (1)\end{matrix}$where E_(P) is the pulse energy in J and D is the beam spot diameter incm. Applicants have found that drilling the same material at the samefluence level with lasers having different pulsewidths results indifferent quality for the bottom copper cladding layer of a targetspecimen. Applicants have determined that a more appropriate parameterfor predicting the bottom copper cladding quality for blind vias is

$\frac{F}{\sqrt{\tau}},$where F is the pulse fluence in J/cm² and τ is the laser pulsewidth innanoseconds.

In view of the foregoing, applicants have developed a model to quantifythe number of pulses used for pad cleaning that depends on a parameterL, which is a function of the laser pulse repetition rate f and thelaser pulsewidth τ. Some relationships between δN, F/√{square root over(τ)}, and L can be expressed as

$\begin{matrix}{L = {{L\left( {f,\tau} \right)} = {\sqrt{\frac{1}{f \cdot \tau}} - \sqrt{\frac{1}{f \cdot \tau} - 1}}}} & (2) \\{{L \cdot \left( {{\delta\; N} - 1} \right)} \approx {\frac{1}{C_{1}} \cdot \left( {\frac{T_{m} - T_{0}}{F/\sqrt{\tau}} - C_{2}} \right)}} & (3)\end{matrix}$where C₁ and C₂ are coefficients related to metal (pad) properties (suchas optical, thermal, and/or mechanical properties) and where T_(m) andT₀ are the melting temperature and the initial temperature of metal pad.

FIG. 2 is a graphical representation of δN versus F/√{square root over(τ)} for different values of the parameter L for an exemplary viadrilling punching process using an imaged UV beam. FIG. 2 shows thatwhen the term F/√{square root over (τ)} is large enough, δN can beminimized to 1.

Based on the foregoing revelations, δN can be predicted for differentlaser parameters. For example, a solid-state UV laser, having availablelaser power of 1.35 watts at the work surface at 50 kilohertz (kHz) todrill a via having a beam diameter of 58 μm, provides a fluence at thework surface of 1.02 J/cm². At a nanosecond pulsewidth such as r=75 ns,F/47=0.12 J/cm²ns^(1/2) and L=0.030647, so δN=20. However, at apicosecond pulsewidth such as τr=0 ps, F/4=10.22 J/cm² ns^(1/2) andL=0.000354; so, at this condition, one can be almost certain that δN=1.

These examples illustrate the improved efficiency in the via drillingprocess that can be obtained by using a picosecond pulsewidthsolid-state UV laser. A solid-state UV laser, having laser output in thepicosecond pulsewidth regime (from 1 ps to 1,000 ps), can allow for suchan exemplary δN=1 process by creating steeper temperature gradients atthe interface of the bulk material and the target pad material of thevia, resulting in more efficient cleaning of the last remaining materialon the target pad. The lower energy put into the target pad decreasesthe chances of thermal damage for small isolated target pads, which arenot directly attached to a circuit trace and therefore cannot dispenseof excess energy through a circuit trace. While some preferredembodiments employ UV laser pulses having a pulsewidth shorter than1,000 ps, some embodiments employ UV laser pulses having a pulsewidthshorter than 500 ps, and some embodiments employ UV laser pulses havinga pulsewidth shorter than 100 ps. Pulsewidths shorter than 1 ps andparticularly in the femtosecond regime may also be employed.

In addition to the bulk material removal and pad cleaning embodimentspresented above, another process of particular interest is via drillingin FR4 and BT resins, either blind vias or through-holes. FR4 may bedifficult to laser-machine for several reasons. First, the material ishighly heterogeneous, particularly with respect to properties governinglaser ablation characteristics such as melting and vaporizationtemperatures. Specifically, the vaporization temperatures of the wovenglass reinforcement and the polymer resin matrix differ greatly. Puresilicon dioxide has melting and vaporization temperatures of 1,970Kelvin (K) and 2,503 K, respectively, while typical organic resins suchas epoxies vaporize at much lower temperatures, on the order of 500 to700 K. This disparity makes it difficult to laser-ablate the glasscomponent while avoiding ablation of too much of the resin surroundingindividual glass fibers or in regions directly adjacent to fiberbundles.

Most FR4 glass cloth is also woven from bundles or “yarns” of individualglass filaments. Filaments are typically 4 to 7 μm in diameter, andyarns range from about 50 μm to several hundred microns in diameter. Theyarns are generally woven in an open-weave pattern, resulting in areasof high glass density where yarns cross each other and areas of low orzero glass density, such as between adjacent bundles. Because thelocations of vias cannot be selected a priori with respect to the weavepattern, the desirable via locations will vary with glass density. Thuslaser micromachining process parameters that work equally well in bothhigh- and low-glass-density regions of the substrate are desirable.Process conditions that are “aggressive” enough to cleanly vaporize allthe glass in high-density regions and at the same time are “mild” enoughto avoid over-etching or removing too much resin or causing excessivepad damage in low-density regions have been difficult to achieve withmost conventional laser processes.

For via drilling in woven reinforced resins, the picosecond pulsewidthsolid-state UV laser can process the material with a smallerheat-affected zone and result in vias with better sidewall quality.FIGS. 3A and 3B show cross sections of through-hole vias drilled inwoven reinforced resin that exhibit respective small and largeheat-affected zones. Vias drilled with picosecond pulsewidth parametersmay exhibit the smaller heat-affected zone similar to that shown in FIG.3A and have less fiber protrusion on the sidewall of the via. Thisquality would be expected when drilling both blind vias andthrough-holes in this material in the picosecond pulsewidth regime. Theincreased fiber protrusion exhibited in FIG. 3B can be a consequence ofa large heat-affected zone such as may be produced by parametersemploying a tens of nanosecond pulsewidth regime.

For woven reinforced resin blind vias and through-hole via drilling, thepicosecond pulsewidth solid-state UV laser process can decrease thethermal diffusion of heat into the sidewalls of the via and result inimproved via sidewall quality. Similarly, for via drilling throughmaterials with a top metal layer, the picosecond pulsewidth solid-stateUV laser can decrease the thermal diffusion of heat into the metal layerand result in better quality cutting and less chance of thermal damageto the metal layer, especially for thin metal layers.

While some embodiments and examples are directed to via drilling, thetechniques are also applicable to other applications of materialremoval, such as the machining of green ceramics, solder pad cleaning,or removal of photoresist material.

Laser machining green ceramics poses concerns similar to those forprocessing FR4 due to the differences in the thermal properties of theorganic binders and the ceramic microparticles. The disparity betweenthe vaporization temperature of the binder (again, on the order of 500K) and the ceramic (3,800 K for Al₂O₃) influences the way material isremoved during laser drilling. Because ceramic has a high vaporizationtemperature, it is quite difficult to remove green ceramic throughdirect melting (at 2,340 K for Al₂O₃) or vaporization of themicroparticles.

The preferred laser micromachining process instead relies upon theexplosive vaporization of the binder material holding the microparticlestogether. When exposed to laser pulses, the binder vaporizes much moreeasily than the ceramic, and the organic vapor is driven to a hightemperature at extremely high heating rates, creating localizedhigh-pressure gas regions in the spaces between microparticles. Thehigh-pressure gas then expands rapidly, disintegrating the green ceramicmaterial. Thus the green ceramic material can be removed while in itssolid state with each laser pulse, at removal rates much higher thancould be obtained by its direct vaporization.

Material removal by explosive vaporization of the binder can be eitheradvantageous or disadvantageous in laser micromachining green ceramics.If the organic vapor pressure is too high or spread across too wide anarea, undesirable effects such as chipping or microcracking can occur.If the high-pressure regions are too localized or not hot enough, poormaterial-removal rates will result. The picosecond pulsewidthsolid-state UV laser process can decrease the thermal diffusion of heatinto the sidewalls of the via and result in improved via sidewallquality in green ceramic materials.

With reference once again to embodiments concerning both blind viadrilling and ablation of protective polymer coverings, anothersignificant concern is that isolated pads may get lifted if the padabsorbs too much laser energy, which disqualifies the process and thevia so formed. Conventional processes are particularly vulnerable tothis pad lifting effect, as the sizes of the features are reducedrelative to the size of the beam doing the ablation. For these smallerfeatures, such as pads having a diameter less than twice the drilled viadiameter and a thickness of typically less than about 18 μm, it isparticularly desirable to reduce the number δN of pad-cleaning pulsesafter the bulk material is removed in order to minimize the energydumped into the pad. This pad lifting effect is expected to become aneven greater concern as feature sizes continue to shrink in the future.

FIGS. 4A and 4B are optical micrographs showing the results of processesto remove solder mask from pads that are approximately the same size asthe laser beam being used to remove the solder mask. Solder mask istypically removed through time-consuming lithographic processes that maysuffer from alignment constraints or through chemical etching processes,and can be difficult to process with typical solid-state lasertechniques.

There are two types of solder mask materials: liquid photoimageablesolder mask (LPISM) and dry film solder mask (DFSM). Typical availableliquid photoimageable solder mask (LPISM) include, but are not limitedto: Coates ImageCure XV501T & XV501TSM, Coates ImageFlex (FlexibleSolder Mask) SV 601T, Enthone DSR 3241 and DSR 3241 GM, Rogers Rflex8080 LP1 and Rflex 8080 LP3 (flexible), Taiyo PSR 4000 BN and 4000 AUS5,Taiyo PSR 9000 (Flexible), or Vantico Probimer 77 LPI Solder Mask.Typical available dry film solder mask (DFSM) include, but are notlimited to: Dupont VACREL 8100, Dupont Flexible Photoimageable Coverlay(PIC) 1000 and 2000, Shipley (Dynachem) DynaMASK 5000, or ShipleyConforMASK 2500.

The pad of the workpiece shown in FIG. 4A received too much energyduring processing and delaminated from the panel. The pad of theworkpiece shown in FIG. 4B did not receive excessive energy from theprocess, so delamination did not occur and the results are acceptable.By having a steeper temperature gradient at the pad with a picosecondpulsewidth solid-state UV laser and a δN=1 process, the amount of energyput into the pad during processing can be reduced and the chance of paddelamination is reduced.

In addition to removal of solder mask material, the UV picosecond lasermachining technique can be employed to remove any resist material withor without photosensitizers. Conventional photoresist materialsgenerally comprise positive photoresists that become soluble whereexposed to light and negative photoresists that become polymerized(insoluble) where exposed to light. Photoresist materials include, butare not limited to, Novolak (M Cresol formaldehyde) or an etch-resistantpoly coating, such as polyisoprene or polymethylisopropenyl ketone.

FIG. 5 is a simplified schematic diagram of an exemplary laser system 10for via formation or solder pad cleaning. With reference to FIG. 5, thelaser system 10 preferably employs a high-average-power pulsedpicosecond laser subsystem 14 that includes a dynamic laser pulsegenerator or oscillator module 12 and an amplification module 16, suchas a DPSS power amplifier.

The dynamic laser pulse generator or oscillator module 12 preferablyemploys a diode-pumped master oscillator to emit oscillator outputpulses having a pulsewidth that is shorter than about 1,000 ps,preferably shorter than about 500 ps, and more preferably shorter than100 ps, at a wavelength shorter than about 400 nanometers (nm), such as266 nm, 351 nm, or 355 nm or other conventionally available solid-stateor fiber laser UV harmonic wavelengths. The oscillator output pulses aredirected into the amplification module 16. The amplification module 16may be a single-pass, multipass, or regenerative DPSS amplifier.Alternatively, the amplification module 16 may be a diode-pumpedrare-earth-doped silica fiber power amplifier. In yet anotherembodiment, the amplification module 16 may be a diode-pumped,rare-earth-doped silica photonic crystal fiber power amplifier.

The oscillator module 12 and the amplification module 16 preferablyemploy Nd-doped lasants as gain materials. A preferred Nd-doped lasantis Nd:GdVO₄, but alternative Nd-doped lasants include, but are notlimited to, Nd:YVO₄, Nd:YLF, Nd:glass, and Nd:YAG. The oscillator module12 and the amplification module 16 may comprise the same or differentlasants with the same or different doping concentrations. The oscillatormodule 12 and the amplification module 16 also preferably employfrequency-selecting elements, prisms, filters, etalons, and/or otherelements well known to skilled practitioners to preferentially producegain at the desired wavelength.

In an exemplary embodiment, an external optical modulator 18, such as anacousto-optic modulator (AOM) or an electro-optic modulator (EOM), canbe triggered to provide laser output 20 a that may contain a singlepulse, multiple independently triggered pulses, or bursts of pulsesselected from a pulse train emitted from the amplification module 16 ofthe picosecond laser subsystem 14. The laser pulses of the laser output20 a have high average power. The optical modulator 18 may be triggereddirectly or indirectly by a system control computer 22, subsysteminterface electronics 24, and/or a modulator control supply 26 as knownto skilled practitioners. The trigger timing may be coordinated, ifdesirable, with the control of a laser power supply 28 directly orindirectly by the system control computer 22 and/or the subsysteminterface electronics 24. Skilled persons will appreciate that usefulAOM modulation techniques are disclosed in U.S. Pat. No. 7,019,891 ofJohnson and can be employed in many embodiments. U.S. Pat. No. 7,019,891is herein incorporated by reference.

In another exemplary embodiment, the oscillator module 12 may comprise apulsed semiconductor laser emitting picosecond pulses. In anotherexemplary embodiment, the oscillator module 12 may comprise a pulsedfiber master oscillator. An exemplary pulsed fiber master oscillator maybe a diode-pumped, Nd-doped or Yb-doped silica fiber master oscillatoremploying a SESAM. Skilled persons will appreciate that otherrare-earth-doped fibers may alternatively be employed and that othermode-locking elements may alternatively be employed.

In another exemplary embodiment, the amplification module 16 may be adiode-pumped, Yb-doped silica fiber master amplifier. In yet anotherexemplary embodiment, the amplification module 16 may be a diode-pumped,Nd-doped silica fiber power amplifier. Skilled persons will appreciatethat other rare-earth-doped fibers may alternatively be employed for theamplification module 16. Skilled persons will appreciate that fibersemploying step index profiles, step index profiles incorporatingpolarization maintaining elements, or air gap profiles may be employed.

With reference to FIG. 6, the laser output 20 a is optionally passedthrough a variety of well-known expansion and/or collimation optics 42,propagated along an optical path 20, and directed by a beam-positioningsystem 30 to impinge laser system output pulse(s) 32 on a desired lasertarget position 34 on a workpiece 52 such as a PWB. An exemplarybeam-positioning system 30 may include a translation stage positionerthat may employ at least two transverse stages 36 and 38 that support,for example, X, Y, and/or Z positioning mirrors 44 and permit quickmovement between the target positions 34 on the same or differentworkpieces 52.

In an exemplary embodiment, the translation stage positioner is asplit-axis system in which a Y stage 36, typically moved by linearmotors along rails 46, supports and moves the workpiece 52, and an Xstage 38, typically moved by linear motors along rails 48, supports andmoves beam-positioning optics such as a fast positioner 50 andassociated focusing lens(es) and/or other optics. The Z dimensionbetween the X stage 38 and the Y stage 36 may also be adjustable. Thepositioning mirrors 44 align the optical path 20 through any turnsbetween the laser subsystem 14 and the fast positioner 50, which ispositioned along the optical path 20. The fast positioner 50 may, forexample, employ high-resolution linear motors, one or more galvanometermirrors, fast-steering mirrors, and/or acousto-optic steering techniquesthat can effect unique or repetitive processing operations based onprovided test or design data. The Y and X stages 36 and 38 and the fastpositioner 50 can be controlled and moved independently or coordinatedto move together in response to panelized or unpanelized data.

The fast positioner 50 may also include or be associated with a visionsystem that can be aligned to one or more fiducials on the surface ofthe workpiece 52. The beam-positioning system 30 can employ conventionalvision or beam-to-work alignment systems that work through a sharedobjective lens, or off-axis, with a separate camera, and that are wellknown to skilled practitioners. In one embodiment, an HRVX vision boxemploying Freedom Library software in a beam-positioning system 30 soldby Electro Scientific Industries, Inc. of Portland, Oreg. is employed toperform alignment between the laser subsystem 14 and the targetpositions 34 on the workpiece 52. Other suitable alignment systems arecommercially available. An exemplary alignment system may employbright-field, on-axis illumination, particularly for specularlyreflecting workpieces such as lapped or polished wafers, but dark-fieldillumination or a combination of dark-field illumination andbright-field illumination may be employed. In addition, thebeam-positioning system 30 may also employ an Abbe error-correctionsystem such as that described in detail in U.S. Pat. No. 6,430,465 ofCutler, the relevant portions of which are herein incorporated byreference.

Many variations of the beam-positioning system 30 are well known toskilled practitioners, and some embodiments of the beam-positioningsystem 30 are described in detail in U.S. Pat. No. 5,751,585 of Cutleret al. The ESI Model 2700 or 5320 micromachining systems available fromElectro Scientific Industries, Inc. of Portland, Oreg. are exemplaryimplementations of the beam-positioning system 30. Other exemplarypositioning systems, such as model series numbers 27xx, 43xx, 44xx, or53xx, 55xx, 56xx, manufactured by Electro Scientific Industries, Inc. ofPortland, Oreg. can also be employed. Those skilled in the art willrecognize that the positioning system can be programmed to utilizetoolpath files that will dynamically position at high speeds the lasersystem output pulses 32 to produce a wide variety of useful via drillingpatterns, which may be either periodic or nonperiodic. Skilled personswill also appreciate that AOM beam-steering techniques disclosed in U.S.Pat. No. 7,019,891 can be used in combination with or substituted forthe fast positioner 50 and/or the beam-positioning system 30.

The laser output 20 a may also be directed through additionalconventional system optical elements that may include, but are notlimited to, nonlinear conversion optics 56, optional correction optics58, and/or optional imaged optics module 62, which may be employed tocontrol the output power and shape the beam profile of the laser pulsesreceived at the target or workpiece surface. Harmonic conversiontechniques employing conventional nonlinear conversion optics 56 toconvert a common fundamental wavelength to a second, third, fourth, orfifth harmonic wavelength are well known to skilled practitioners.

With reference to FIG. 7, the optional imaged optics module 62 mayinclude an optical element 64, a lens 66, and an aperture mask 68 placedat or near the beam waist created by the optical element 64 to block anyundesirable side lobes and peripheral portions of the beam so that aprecisely shaped spot profile is subsequently imaged onto the worksurface. In an exemplary embodiment, the optical element 64 is adiffractive device or lens, and the lens 66 is a collimating lens to addflexibility to the configuration of the laser system 10.

Varying the size of the aperture to match the properties of the opticalelement 64 can control the edge sharpness of the spot profile to producea size-specified, sharper-edged intensity profile that should enhancethe alignment accuracy. In addition, with this arrangement, the shape ofthe aperture can be precisely circular or can be changed to rectangular,elliptical, or other noncircular shapes that can be aligned parallel orperpendicular to a cutting direction. The aperture mask 68 mayoptionally be flared outwardly at its light-exiting side. For UV laserapplications, the aperture mask 68 in the imaged optics module 62preferably comprises sapphire. Skilled persons will appreciate that theaperture mask 68 can be used without the optical element 64 and the lens66.

In an alternative embodiment, optical element 64 includes one or morebeam-shaping components that convert laser pulses having a raw Gaussianirradiance profile into shaped (and focused) pulses that have anear-uniform “top hat” profile, or particularly a super-Gaussianirradiance profile, in proximity to the aperture mask 68 downstream ofthe optical element 64. Such beam-shaping components may includeaspheric optics or diffractive optics. In one embodiment, the lens 66comprises imaging optics useful for controlling beam size anddivergence. Skilled persons will appreciate that a single imaging lenscomponent or multiple lens components could be employed. Skilled personswill also appreciate, and it is preferred, that shaped laser output canbe employed without using the aperture mask 68.

In one embodiment, the beam-shaping components include a diffractiveoptic element (DOE) that can perform complex beam shaping with highefficiency and accuracy. The beam-shaping components not only transformthe Gaussian irradiance profile to a near-uniform irradiance profile,but they also focus the shaped output to a determinable or specifiedspot size. Although a single-element DOE is preferred, skilled personswill appreciate that the DOE may include multiple separate elements suchas the phase plate, and transform elements disclosed in U.S. Pat. No.5,864,430 of Dickey et al., which also discloses techniques fordesigning DOEs for the purpose of beam shaping. The shaping and imagingtechniques discussed above are described in detail in U.S. Pat. No.6,433,301, the relevant portions of which are herein incorporated byreference.

A laser power control module 70 may be employed to achieve laser pulsepower control by using modulation methods including, but not limited to,diode pump modulation or external modulation (such as with an externallaser power controller 60, including, but not limited to, AOMs or EOMsor motorized polarization rotation optics positioned along the opticalpath 20) or a combination thereof. In addition, one or morebeam-detection devices 54, such as photodiodes, may be downstream of thelaser power controller 60, such as aligned with a positioning mirror 44that is adapted to be partly transmissive to the wavelength of the laseroutput 20 a. The beam-detection optics and electronics may be associateddirectly or indirectly with the laser power control module 70, and/ormay be directly or indirectly in communication with the system controlcomputer 22 and/or the subsystem interface electronics 24, and/or may beused to sample modulated laser output 20 a and generate correctionsignals to the modulators and/or other system optical elements toproduce stable modulated output having parameters desirable forprocessing the workpiece 52. Conventional power-control techniques areknown to skilled practitioners. Some exemplary AOM power-controltechniques are disclosed in U.S. Pat. No. 7,019,891.

Preferred pulse repetition frequencies employed range from 50 kHz to 10megahertz (MHz). In many cases, pulse repetition frequencies smallerthan 1 MHz are preferred. Certain applications may, however, employpulse repetition frequencies in the range from 10 MHz to 100 MHz.Typical focused spot sizes employed range from 10 μm to 100 μm. Certainapplications may, however, employ spot sizes in the range from 1.5 μm to10 μm.

Skilled persons will appreciate that the laser parameters employed forpad cleaning may also be used for bulk processing. Alternatively, thelaser parameters for pad cleaning may be different from those used forbulk cleaning. In some embodiments, the fluence employed for bulkremoval is maintained at about the value for pad cleaning, but thepulsewidth for pad cleaning is changed (significantly decreased) todecrease the number of pulses and decrease the amount of time employedfor pad cleaning. Other laser parameters may be changed as well betweenthe bulk processing and pad cleaning steps. Such parameters may include,but are not limited to, wavelength, energy per pulse, repetition rate,or spot size.

Furthermore, skilled persons may recall that U.S. Pat. No. 5,841,099 ofOwen et al. and U.S. Pat. No. 6,407,363 of Dunsky et al., which areherein incorporated by reference, disclose techniques for two-step,single-pass processing of blind and through-hole vias by employing afirst set of laser parameters to process an overlying metal layer and asecond set of laser parameters to process the bulk material. Inparticular, U.S. Pat. No. 5,841,099 of Owen et al. discloses changing(increasing) the repetition rate and/or changing (increasing) the spotsize after processing the overlying metal layer so that the bulkmaterial effectively receives less energy per pulse than the overlyingmetal layer.

Similarly, U.S. Pat. No. 6,407,363 of Dunsky et al. discloses focus- andspot-size-changing techniques employed to control the irradiance used toform the via in order to enhance via quality as well as to provide asingle-pass, two-step method for processing the metal layer and thenprocessing the bulk material. Such techniques can be accomplished withthe use of a deformable mirror, but may also be accomplished with theuse of an AOM.

Skilled persons will appreciate that the techniques disclosed may beemployed to implement a three-step via drilling process such thatdifferent laser parameters are employed to process the overlying metallayer, to remove the bulk material, and to clean the pad. For example,the metal layer may be processed at a first set of parameters, therepetition rate and/or spot size may be changed after the metal layer isremoved to provide a second set of parameters, and then the pulsewidthmay be decreased to provide a third set of parameters for cleaning thepad material. Additionally, different wavelengths may be used for any ofthe steps. These changes may be implemented in a single laser or by twoor more lasers.

Skilled persons will appreciate that the pad cleaning parameters neednot be constrained by typical fluence threshold limitations, becauseremoval of a small amount of the underlying metal layer is acceptableand may be desirable to ensure that the pad surface is completely clean.

Skilled persons will also appreciate that the bulk material removal andthe pad-cleaning process may employ outward- or inward-spiraling orconcentric-circle processing techniques or any variation of loop-profileprocessing techniques, such as those disclosed in U.S. Pat. No.6,407,363 of Dunsky et al., whenever the via size or pad size is largerthan the spot size. Similarly, trepanning techniques can be employed tocreate through-holes. For these applications, typical bite sizes mayrange from about 1 nm to about 15 μm. Typical scanning velocities mayrange from about 10 to about 1,000 millimeters per second.

As presented in the various embodiments and examples, there are for avariety of laser micromachining applications both throughput and qualitybenefits for using UV image shaped laser output employing pulses havinga pulsewidth in the picosecond regime. In particular, a picosecondpulsewidth solid-state UV laser produces steeper thermal gradientsrequiring less energy for cleaning the last remaining layer of material,solder mask, or resist material on the target pad. The lower energy putinto the target pad results in more efficient processing and less chanceof thermal damage for small isolated target pads. For via drilling in orthrough woven reinforced resin, these laser output parameters candecrease the thermal diffusion of heat into the sidewalls of the via, sosidewall quality is improved. For via drilling through materials with atop metal layer, the picosecond pulsewidth regime can decrease thethermal diffusion of heat into the metal layer and result in betterquality cutting and less chance of thermal damage to the metal layer,especially for thin metal layers.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof. Thescope of the present invention should, therefore, be determined only bythe following claims.

1. A method for increasing laser via drilling throughput by minimizing atotal number of pulses used to drill a via having desirable operationalcharacteristics, wherein the total number N of pulses includes a bulknumber N₀ of bulk removal pulses employed to remove bulk material toform the via with a laser removal-bulk material interaction and a bottomsurface cleaning number δN of bottom surface cleaning pulses employed toclean a bottom surface of the via with a laser cleaning-materialinteraction, comprising: generating, from a laser, laser output having awavelength shorter than 400 nanometers, a fluence F, and at least onelaser pulse having a pulsewidth τ, shorter than 1000 picoseconds, forcleaning the bottom surface of the via, where δN has a relationship toF/τ^(1/2) and such that δN/N₀ is less than or equal to 1; and directingthe laser output from the laser to a target position to remove a majorportion of the bulk material and clean the bottom surface of a via, suchthat laser pulses of the laser output for removing the major portion ofthe bulk material and for cleaning the bottom surface of the via have awavelength shorter than 400 nanometers and a pulsewidth shorter than1000 picoseconds.
 2. The method of claim 1 in which the relationshipbetween δN and F/τ^(1/2) satisfies the equation:${{L \cdot \left( {{\delta\; N} - 1} \right)} \approx {\frac{1}{C_{1}} \cdot \left( {\frac{T_{m} - T_{0}}{F/\sqrt{\tau}} - C_{2}} \right)}},$in which L=(1/fτ)^(1/2)−(1/fτ−1)^(1/2), and f is the repetition rate. 3.The method of claim 1 in which δN has a relationship to L, whereL=(1/fτ)^(1/2)−(1/fτ−1)^(1/2) and f is the repetition rate.
 4. Themethod of claim 1 in which multiple vias are formed and cleaned in asingle pass, such that the laser is directed by a beam positioningsystem to address each target position once to perform both operationsof bulk material removal and cleaning.
 5. The method of claim 1 in whichthe laser output comprises a wavelength of about 355 nanometers or 351nanometers.
 6. The method of claim 1 in which the laser output has arepetition rate between 10 megahertz and 100 megahertz.
 7. The method ofclaim 1 in which the laser pulse has a pulsewidth that is shorter than500 picoseconds.
 8. The method of claim 1, further comprising: employingimage-shaping optics to shape the laser output.
 9. The method of claim 1in which the via is drilled in a printed wiring board.
 10. The method ofclaim 1 in which the bulk material includes a homogenous film, aparticulate-filled resin, a polyimide, or a fiber-reinforced polymer.11. The method of claim 10 in which the bulk material includes a metalcladding.
 12. The method of claim 11 in which the metal cladding is lessthan about 18 microns thick.
 13. The method of claim 1 in which thebottom surface material includes a metal.
 14. The method of claim 1 inwhich the laser output is generated by a solid-state laser or a fiberlaser.
 15. The method of claim 1 in which the laser output is employedin a laser punching process.
 16. The method of claim 1 in which the bulkmaterial includes a fiber-reinforced polymer and the via has sidewallsexhibiting minimal fiber protrusion.
 17. The method of claim 1 in whichthe laser output has a repetition rate that is less than or equal to 10megahertz.
 18. The method of claim 1 in which the laser output has arepetition rate that is less than or equal to 1 megahertz.
 19. Themethod of claim 1 in which the bulk material removal and bottom surfacecleaning are performed at about the same fluence but at differentpulsewidths.
 20. The method of claim 1 in which bottom surface cleaningremoves less than a 2-micron thickness of material.
 21. The method ofclaim 1 in which the via drilling is accomplished by a punching process.22. The method of claim 1 in which the via drilling is accomplished by atrepanning, spiraling, or looping process.
 23. The method of claim 1 inwhich a workpiece at the target position includes an overlying metallayer, a bulk material positioned beneath the overlying metal layer, andan underlying metallic layer or pad positioned beneath the bulkmaterial, and in which the top metal layer is removed with a first setof laser parameters, the bulk material is removed with a second set oflaser parameters, and the underlying metallic layer is cleaned with athird set of laser parameters, wherein the first, second, and third setsof parameters are different.
 24. A method for laser drilling a blind viain a workpiece including a bulk material and an underlying bottomsurface material wherein a laser-bulk material interaction determines abulk set of optimal laser processing parameters for efficiently removinga major portion of the bulk material to form the blind via and wherein alaser cleaning-material interaction determines a cleaning set of optimallaser processing parameters for efficiently cleaning the bottom surfacewith a fewest number of bottom surface-cleaning pulses employed to cleanthe bottom surface of the blind via without causing operational damageto the bottom surface of the blind via, such that the cleaning set ofoptimal laser processing parameters is less efficient for removing bulkmaterial than is the bulk set of optimal laser processing parameters,comprising: determining for a wavelength shorter than 400 nm optimalranges of fluence, pulsewidth, and repetition rate within the cleaningset for minimizing the number of laser pulses to clean the bottom of ablind via, employing a solid-state laser to generate laser output at awavelength shorter than 400 nanometers having parameters within thecleaning set of optimal laser processing parameters, the laser outputincluding multiple laser pulses having pulsewidths shorter than 500picoseconds; employing image-shaping optics to shape the laser output;and directing the multiple laser pulses of the laser output at a targetposition to remove the major portion of the bulk material to form theblind via in the workpiece and to clean the bottom surface material. 25.The method of claim 24 in which the bulk material includes a homogenousfilm, a particulate filled resin, a polyimide, or a fiber-reinforcedpolymer.
 26. The method of claim 25 in which the bulk material includesa metal cladding.
 27. The method of claim 26 in which the metal claddingis less than about 18 microns thick.
 28. The method of claim 24 in whichthe bottom surface material includes a metal.
 29. The method of claim 24in which the bulk material includes a resistive material.
 30. The methodof claim 24 in which the laser output has a repetition rate that is lessthan or equal to 10 megahertz.
 31. The method of claim 24 in whichmultiple vias are formed and cleaned in a single pass, such that thelaser is directed by a beam positioning system to address each targetposition once to perform both operations of bulk material removal andcleaning.
 32. The method of claim 24 in which the laser output has arepetition rate between 10 megahertz and 100 megahertz.